This invention relates in general to angular rate sensors and in particular to an apparatus and a method for performing a diagnostic test upon an angular rate sensor.
Angular rate sensors are widely used in many commercial applications, such as, for example, attitude control systems for automobiles, a gyroscope for a navigation system included in a moving object or a hand-shake compensating system for video cameras. Angular rate sensors measure the rate of rotation of a body about its three principle axes. The rotational movement is typically referred to as yaw, pitch and roll which are related to vertical, transverse and longitudinal axes, respectively.
A simplified angular rate sensor element 10 is illustrated in FIGS. 1 and 2 as a tuning fork. Such sensors typically include a pair of vibrating elements, that are shown as tines 11 and 12 in FIG. 1. The lower ends of the tines 11 and 12 are connected by an output shaft 3. The tines 11 and 12, which function as proof masses, are driven in opposite directions in the plane of the drawing by electrostatic drive motors 14 and 15. The tines 11 and 12 vibrate in the directions shown by the small arrows labeled 16 and 17. when an angular rate, illustrated by the circular arrow 18, is applied to the sensor element 10 about an axis 19 in the plane of the sensor element 10, the tines 11 and 12 are caused to oscillate out of the plane by a Coriolis force due to Coriolis effect, as shown in FIG. 2 by the small arrows labeled 20 and 21. The resulting out-of plane oscillation motion amplitude, which is proportional to the input angular rate, is detected and measured by capacitive or electrostatic pickoff plates 22 that are located adjacent to the upper ends of the tines 11 and 12. While a simplified sensor element 10 has been shown in FIGS. 1 and 2, it will be appreciated that angular rate sensors typically include one, two, four or any plurality of tuning forks in a unitary system.
The size of angular rate sensors has been reduced by the development of resonant Micro Electro Mechanical System (MEMS) angular rate sensors that feature micromachined mechanical components and integrated support electronics. Thus, MEMS includes the concept of integration of microelectronics and micromachining. These devices can be fabricated from various materials, such as, for example, silicon, quartz and ceramics. Examples of successful MEMS devices include inkjet printer cartridges, accelerometers that deploy car airbags and miniature robots.
A schematic diagram for a typical MEMS angular rate sensor 23 is illustrated in FIG. 3. Components shown in FIG. 3 that are similar to components shown in FIGS. 1 and 2 have the same numerical identifiers. Additional capacitive or electrostatic pickoff plates 24 and 25 have been added to measure amplitude and frequency of the oscillation of the tines 11 and 12 within the plane of the sense element 10. The information obtained from pickoff plates 24 and 25 is fed back through the lines labeled 28 and 29 to a closed loop sense element drive circuit 30. While the lines 28 and 29 are shown as single wires, it will be appreciated that the circuit has been simplified for clarity and that multiple wires or traces may actually be used. The sense element drive circuit 26 is conventional and typically includes an oscillator with automatic gain and frequency control (not shown) that receives the feedback from the pickoff plates 24 and 25 to assure that the linear motion is provided to sense element 10. Sense element drive signals are supplied to the electrostatic drive motors 14 and 15 by the output lines 28A and 29A.
Analog data from the Coriolis force pickoff plates 22 is supplied through the lines labeled 32 and 34 to an open loop signal conditioning circuit 36. While the lines 32 and 34 are as single wires, it will appreciated that the drive circuit has been simplified for clarity and that multiple wires or traces may actually be used. The signal conditioning circuit 36 generates an output signal that is proportional to the angular rate on an output line 38. The output signal may be either an analog or a digital signal. The output line 38 is connected to an input port of a microprocessor (not shown).
A block diagram 40 for typical signal conditioning circuit 36 is illustrated in FIG. 4. Additionally, the sense element 10 is shown in block form. Thus, an input angular rate xcfx89in is applied to a proof mass, m, or the tines 11 and 12 in the illustrative example, in block 42. The proof mass m responds to the angular rate xcfx89in with an input force Fin that is applied to the sense elements, or the Coriolis force pickoff plates 22, in block 44. The in input force Fin is converted to voltage, V, in block 46 and supplied to the signal conditioning circuit 36. The voltage V is amplified and any offset is cancelled in block 48. The amplified signal is filtered in block 50 and then converted to a digital signal by a quantizer in block 52. Alternately, the output of the filter in block 50 can be directly used as an analog signal, in which case the quantizer in block 52 is omitted from FIG. 4.
It is known to test angular rate sensors as illustrated in FIG. 5. In FIG. 5, the output of the filter block 50 is supplied directly as an analog signal to an analog input pin 53 of an Electronic Control Unit (ECU) 54. The ECU 54 has a test output pin 55 that is connected to a test signal generator 56. The test signal generator 56 generates an analog test signal when the ECU output pin 55 changes state, such as, for example, goes from zero voltage to a high value, which is typically five volts. The analog test signal is injected at point 57 to the input of the signal conditioning circuit 36. The resulting analog output signal is converted to a digital output signal by an analog to digital converter 58 within the ECU 54. The digital output signal is supplied to a comparator 59 that compares the output signal value to an expected value that corresponds to the test signal. If the output signal value is different from the expected value, an error flag is set to indicate that the signal conditioning circuit is malfunctioning. Alternately, the difference between the output signal and the expected value are compared to a predetermined threshold. If the difference exceeds the threshold, the error flag is set. While the test signal generator 56 is shown in FIG. 5 as a separate component, it will be appreciated that the circuit can be included in the ECU 54 (not shown).
The above test exercises all components of the angular rate sensor but the MEMS element. Accordingly, it would be desirable to provide an angular rate sensor that includes a functional test of the sense element 10.
This invention relates to an apparatus and a method for performing a diagnostic test upon a angular rate sensor.
The present invention contemplates a device for measuring a angular rate comprising a sensor element with a first feedback control device connected to the sensor element, the first feedback control device operative to resonate the sensor element. The device also includes a second feedback control device connected to the sensor element, the second feedback control device operative to sense the presence of a secondary mode signal generated by the sensor element in response to a Coriolis force and to generate a null signal that cancels said secondary mode signal. The second feedback control device also generates an output signal that is proportional to the null signal.
The angular rate measuring device further includes a test device connected to the second feedback control device, the test device being operative to inject a test signal into the second feedback loop such that the test signal is passed through the sense element. Accordingly, the output signal will be proportional to the test signal. The test device is further operative to compare the resulting output signal to the previous output signal and to set an error flag if the output signal has not changed as a result of the injection of the test signal. The test device also sets an error flag if the resulting output signal exceeds a first predetermined threshold or if the change in the output signal exceeds a second predetermined threshold.
The invention also contemplates a method for testing an angular rate measuring device comprising the steps of providing a sensor element and a first feedback control device connected to the sensor element. The first feedback control device is operative to resonate the sensor element. The angular rate measuring device also includes a second feedback control device connected to the sensor element. The second feedback control device is operative to sense the presence of a secondary mode signal generated by the sensor element in response to a Coriolis force and to generate a null signal that cancels the secondary mode signal. The second feedback control device also generates an output signal that is proportional to the null signal.
The method includes first resonating the sensor element and subsequently injecting a test signal into the second feedback control device that is passed through the sense element. Accordingly, the output signal is proportional to the test signal. The method then compares the resulting output signal to the preceding output signal and sets an flag if the output signal has not changed in response to the test signal. Additionally, an error flag is set if the output signal resulting from the test signal exceeds a first determined threshold or if the change in the output signal exceeds a second predetermined threshold.
Various objects and advantages of this invention will become apparent to those skilled in the art from the following detailed description of the preferred embodiment, when read in light of the accompanying drawings.